1. Field of the Disclosure
The present invention relates to novel interconnect structures including coaxial vias, and methods for fabricating same.
2. Description of the Related Art
Driven by an ever greater demand for miniaturization of ever more complex electronic components, consumer electronics such as computing and telecommunication devices are becoming more integrated. This has created a need for support structures such as IC substrates and IC interposers that have a high density of multiple conductive layers and vias that are electrically insulated from each other by a dielectric material.
The general requirement for such support structures is reliability and appropriate electrical performance, thinness, stiffness, planarity, good heat dissipation and a competitive unit price.
Of the various approaches for achieving these requirements, one widely implemented manufacturing technique that creates interconnecting vias between layers uses lasers to drill holes through the subsequently laid down dielectric substrate through to the latest metal layer for subsequent filling with a metal, usually copper, that is deposited therein by a plating technique. This approach to creating vias is sometimes referred to as ‘drill & fill’, and the vias created thereby may be referred to as ‘drilled & filled vias’.
There are, however, a number of disadvantages with the drilled & filled via approach:
Since each via is required to be separately drilled, the throughput rate is limited, and the costs of fabricating sophisticated, multi-via IC substrates and interposers becomes prohibitive.
In large arrays it is difficult to produce a high density of high quality vias having different sizes and shapes in close proximity to each other by the drill & fill methodology.
Furthermore, laser drilled vias have rough side walls and taper inwards through the thickness of the dielectric material. This tapering reduces the effective diameter of the via. It may also adversely affect the electrical contact to the previous conductive metal layer especially at ultra small via diameters, thereby causing reliability issues.
The side walls are particularly rough where the dielectric being drilled is a composite material comprising glass or ceramic fibers in a polymer matrix, and this roughness may create additional stray inductances.
The filling process of the drilled via holes is usually achieved by copper electroplating. This metal deposition technique may result in dimpling, where a small crater appears at the top of the via. Alternatively, overfill may result, where a via channel is filled with more copper than it can hold, and a domed upper surface that protrudes over the surrounding material is created. Both dimpling and overfill tend to create difficulties when subsequently stacking vias one on top of the other, as required when fabricating high-density substrates and interposers.
Large via channels are difficult to fill uniformly, especially when they are in proximity to smaller vias within the same interconnecting layer of the interposer or IC substrate design.
Laser drilling is best for creating round via channels. Although slot shaped via channels may be fabricated by laser milling, nevertheless, the range of geometries that may be fabricated by ‘drill & fill’ is somewhat limited. Fabrication of vias by drill & fill is expensive and it is difficult to evenly and consistently fill the via channels created thereby with copper using the relatively, cost-effective electroplating process.
Although the range of acceptable sizes and reliability is improving over time, the disadvantages described hereinabove are intrinsic to the drill & fill technology and are expected to limit the range of possible via sizes.
An alternative solution that overcomes many of the disadvantages of the drill & fill approach, is to fabricate vias by depositing copper or other metal into a pattern created in a photoresist, using a technology otherwise known as ‘pattern plating’.
In pattern plating, a seed layer is first deposited. Then a layer of photoresist is laid down over the seed layer and subsequently exposed to create a pattern, which is selectively removed to leave trenches that expose the seed layer. Via posts are created by depositing copper into the photoresist trenches. The remaining photoresist is then removed, the seed layer is etched away, and a dielectric material that is typically a polymer impregnated glass fiber mat, is laminated thereover and therearound to encase the via posts. Various techniques and processes, such as mechanical grinding, polishing, chemical etching and chemical mechanical polishing may then be used to remove part of the dielectric material and to expose the top of the via posts, allowing building up the next metal layer. Thin process may be referred to as planarizing or as thinning. Subsequent layers of metal conductors and via posts may be deposited there onto by repeating the process to build up a desired multilayer structure.
In an alternative but closely linked technology, known hereinafter as ‘panel plating’, a continuous layer of metal or alloy is deposited onto a substrate. A layer of photoresist is laid on top of the continuous layer, and a pattern is developed therein. The pattern of developed photoresist is stripped away, selectively exposing the metal thereunder, which may then be etched away. The undeveloped photoresist protects the underlying metal from being etched away, and leaves a pattern of upstanding features and vias.
After stripping away the undeveloped photoresist, a dielectric material, such as a polymer impregnated glass fiber mat, may be laminated around and over the upstanding copper features and/or via posts.
The via layers created by pattern plating or panel plating methodologies described above are typically known as via post layers and feature layers. Copper is a preferred metal for both layers.
It will be appreciated that the general thrust of the microelectronic evolution is directed towards fabricating ever smaller, thinner and lighter and more powerful products having high reliability. The use of thick, cored interconnects, prevents ultra-thin products being attainable. To create ever higher densities of structures in the interconnect IC substrate or interposer, ever more layers of ever smaller connections are required. Indeed, sometimes it is desirable to stack components on top of each other.
If plated, laminated structures are deposited on a copper or other appropriate sacrificial substrate, the substrate may be etched away leaving free standing, coreless laminar structures. Further layers may be deposited on the side previously adhered to the sacrificial substrate, thereby enabling a two sided build up, which minimizes warping and aids the attaining of planarity.
One flexible technology for fabricating high density interconnects is to build up pattern or panel plated multilayer structures consisting of metal vias or features in a dielectric matrix. The metal may be copper and the dielectric may be a fiber reinforced polymer, typically a polymer with a high glass transition temperature (Tg) is used, such as polyimide, for example. These interconnects may be cored or coreless, and may include cavities for stacking components. They may have odd or even numbers of layers. Enabling technology is described in previous patents issued to Amitec-Advanced Multilayer Interconnect Technologies Ltd.
For example, U.S. Pat. No. 7,682,972 to Hurwitz et al. titled “Advanced multilayer coreless support structures and method for their fabrication” describes a method of fabricating a free standing membrane including a via array in a dielectric, for use as a precursor in the construction of superior electronic support structures. The method includes the steps of fabricating a membrane of conductive vias in a dielectric surround on a sacrificial carrier, and detaching the membrane from the sacrificial carrier to form a free standing laminated array. An electronic substrate based on such a free standing membrane may be formed by thinning and planarizing the laminated array, followed by terminating the vias. This publication is incorporated herein by reference in its entirety.
U.S. Pat. No. 7,669,320 to Hurwitz et al. titled “Coreless cavity substrates for chip packaging and their fabrication” describes a method for fabricating an IC support for supporting a first IC die connected in series with a second IC die; the IC support comprising a stack of alternating layers of copper features and vias in insulating surround. The first IC die is bondable onto the IC support, and the second IC die is bondable within a cavity inside the IC support, wherein the cavity is formed by etching away a copper base and selectively etching away built up copper. This publication is incorporated herein by reference in its entirety.
U.S. Pat. No. 7,635,641 to Hurwitz et al. titled “integrated circuit support structures and their fabrication” describes a method of fabricating an electronic substrate comprising the steps of: (A) selecting a first base layer; (B) depositing a first adhesive etchant resistant barrier layer onto the first base layer; (C) building up a first half stack of alternating conductive layers and insulating layers, the conductive layers being interconnected by vias through the insulating layers; (D) applying a second base layer onto the first half stack; (E) applying a protective coating of photoresist to the second base layer; (F) etching away the first base layer; (G) removing the protective coating of photoresist; (H) removing the first adhesive etchant resistant barrier layer; (I) building up a second half stack of alternating conductive layers and insulating layers, the conductive layers being interconnected by vias through the insulating layers, wherein the second half stack has a substantially symmetrical lay up to the first half stack; (J) applying an insulating layer onto the second hall stack of alternating conductive layers and insulating layers, (K) removing the second base layer, and (L) terminating the substrate by exposing ends of vias on outer surfaces of the stack and applying terminations thereto. This publication is incorporated herein by reference in its entirety.